Exclusive Generation\nof Single-Atom Sulfur for Ultrahigh\nQuality Monolayer MoS<sub>2</sub> Growth

Preparation of high-quality two-dimensional\n(2D) transition\nmetal\ndichalcogenides (TMDCs) is the precondition for realizing their applications.\nHowever, the synthesized 2D TMDCs (e.g., MoS<sub>2</sub>) crystals\nsuffer from low quality due to the massive defects formed during the\ngrowth. Here, we report single-atom sulfur (S<sub>1</sub>) as a highly\nreactive sulfur species to grow ultrahigh-quality monolayer MoS<sub>2</sub>. Derived from battery waste, sulfurized polyacrylonitrile\n(SPAN) is found to be exclusive and efficient in releasing S<sub>1</sub>. The monolayer MoS<sub>2</sub> prepared by SPAN exhibits an ultralow\ndefect density of ∼7 × 10<sup>12</sup> cm<sup>–2</sup> and the narrowest photoluminescence (PL) emission peak with full-width\nat half-maximum of ∼47.11 meV at room temperature. Moreover,\nthe statistical resonance Raman and low-temperature PL results further\nverify the significantly lower defect density and higher optical quality\nof SPAN-grown MoS<sub>2</sub> than those of the conventional S-powder-grown\nsamples. This work provides an effective approach for preparing ultrahigh-quality\n2D single crystals, facilitating their industrial applications.